The invention relates to a method for melting pig iron using recirculated gas from the reduction region.
According to the prior art, various methods for the melting of pig iron are known. For example, GB 883998 A describes, in blast furnaces, injecting various carbon-containing gases, such as natural gas or coke oven gas, via the tuyers or at the bosh level, in order thereby to save coke and increase the efficiency of the plant. An injection of blast furnace gas from a conventional blast furnace operated with hot blast air is not economical because of the high content of carbon dioxide and nitrogen and the low content of hydrogen.
DE 19 39 354 A describes the reforming of blast furnace gas from a blast furnace with hydrocarbon fuel in an underfired external reaction system and the recirculation of this blast furnace gas into the blast furnace. This has the disadvantage of the additional outlay required in terms of energy for the underfiring of the heater and also the very high outlay for the high-temperature heat exchanger. Furthermore, during heating, disadvantages arise because of what is known as “metal dusting corrosion” caused by carbon monoxide and hydrogen in the reduction gas.
DE 2 261 766 describes the recirculation of blast furnace gas from a blast furnace operated with oxygen instead of hot blast air. After the scrubbing of the blast furnace gas, the carbon dioxide is removed, and is introduced to the blast furnace via tuyers on the bosh or hearth. Furthermore, the operation of a blast furnace with oxygen and the recirculation of the blast furnace gas and also the use of a CO2 removal plant are also described in DE 3702875 C1. WO 2004/052510 A3 describes the operation of a blast furnace with hot blast air and the recirculation of blast furnace gas and also the removal of carbon dioxide and nitrogen. In these embodiments, however, there are substantial disadvantages in the considerable outlays in terms of investment and operating costs for the removal of carbon dioxide and nitrogen from the blast furnace gas and also in the necessary heating of the reduction gas, this giving rise, again, to problems with metal dusting corrosion. Moreover, the residual gas from CO2 removal has to be delivered for utilization or retreatment on account of the low calorific value. In this case, further, the carbon dioxide is locked out in the blast furnace gas, thus leading to adverse CO2 emissions, because it is not reformed again into reduction gas. Additional losses of reductants therefore occur in the residual gas.
The same disadvantages arise also in a method according to DE 10 2004 036 767 A1 which is interpreted as the nearest prior art. This document describes a method for pig iron production in a blast furnace operated with oxygen and recirculated blast furnace gas, with the addition of hydrocarbons, although, once again, the previous removal of carbon dioxide is required.